Time dispersive characteristics (also called “delay spread characteristics”) between sub-carriers in a mobile communication system employing a plurality of sub-carriers have been determined according to communication environments of a cell managed by a base station (i.e., Node-B). For example, a large amount of time delay occurs in an urban district whereas a small amount of time delay occurs in a vast expanse of land. However, frequency dispersive characteristics (also called “Doppler spread”) between sub-carriers depend on a channel variation affected by a speed between a transmitter and a receiver.
A representative example of the mobile communication system employing a plurality of sub-carriers is an Orthogonal Frequency Division Multiplexing (OFDM) system. The OFDM system inserts various-sized guard intervals to reflect various time dispersive characteristics affected by communication situations. There are two kinds of guard intervals, a cyclic prefix (CP) and a cyclic postfix (CP). Theoretically, if all multi-taps are arrived in the cyclic prefix, there is no inter-symbol interference (ISI). For example, the CP length of the Orthogonal Frequency Division Multiplexing Access (OFDMA) system prescribed in the IEEE 802.16 system is selected from 2.85 μs˜22.8 μs at a bandwidth of 10 MHz.
A conventional OFDM system has no additional solution for reflecting various frequency dispersive characteristics affected by communication situations, because a cell includes a plurality of mobile terminals (or user equipments) of different speeds, so that the conventional OFDM system is unable to set a focal point on only a specific mobile terminal from among them. As a result, a spacing between sub-carriers (hereinafter referred to as a sub-carrier spacing) must be uniformly established on the basis of the highest speed capable of being supported by a corresponding communication system.
However, if the sub-carrier spacing is fixed at a specific value and a mobile-terminal speed is higher than a reference speed, inter-carrier interference increases by the above-mentioned frequency dispersive characteristics, and orthogonal characteristics of a signal is broken, so that the reliability of data transmission may be deteriorated. Otherwise, if the mobile-terminal speed is lower than a reference speed, unnecessary resources are allocated so that the utility of frequency resources may be deteriorated.